Like Bubba says, it really depends upon:
1. how much you want to spend
2. how big you want it to be
We have built ponds using pondliner and most recently with several layers of (cheap) 10 mil thick plastic sandwiched with sandy loam and topped with sandy loam. So far it is holding up well.
In our experience a man-made duck pond will be:
1. muddy as all get out
2. plant-less, unless you can protect the plants from the ducks.
2' deep is sufficiently deep, in our experience. Two ducks won't need a very wide pond, but you never know if you'll end up getting more ducks . . . .
The rule of thumb with pond building is to always make it as big as you can.